A quantitative autoradiographic method for the determination of local rates of protein synthesis in brain in vivo has been developed with L-[1- 14C]leucine as the tracer amino acid. A four-compartment model for the behavior of leucine in brain has been analyzed, and an equation has been derived that defines the rates of amino acid incorporation into protein in terms of the time course of the plasma leucine specific activity, final tissue concentrations of 14C, and lambda i, the steady state ratio of the distribution volumes for the labeled and unlabeled leucine in the immediate precursor pool for protein synthesis. All of the variables are directly measurable except for lambda i which must be determined separately. Lambda i is a measure of the fraction of leucine in the precursor pool for protein synthesis that is derived from plasma; the remainder is derived from protein degradation. The value of lambda i for the brain as a whole in conscious, adult, male rats has been experimentally determined to be 0.58. Lambda i does show some regional variation but all brain regions examined were within 14% of the whole brain value. During development lambda i is 10% lower than it is in the adult. In the adult, lambda i increases under light thiopental anesthesia and decreases in regenerating cranial nerve nuclei. These results show that, in general, the amino acid precursor pool for protein synthesis is partially derived from protein degradation. The fraction of the pool coming from protein degradation is not uniform in all brain regions and changes with experimental conditions. Studies carried out on applications of the method that examine some basic neurobiological questions include: 1) barbiturate & ketamine anesthesia; 2) slow wave sleep; 3) electrical stimulation; 4) hypothyroidism & hyperthyroidism; 5) regeneration and 6) chronic drug administration.